bindgen should use the RUSTFMT environment variable if it wasn't set with with_rustfmt

[glandium]

It seems absurd that every single build script that depends on bindgen and doesn't disable rustfmt has to handle getting the environment variable and use with_rustfmt on its own. The net result is that in Firefox, only stylo uses the RUSTFMT environment variable, and baldrdash, coreaudio-sys and js all ignore it.

[emilio]

Sure, that's fair. Should be a pretty easy change.